1. Field of the Invention
The present invention relates to a digital image forming apparatus such as a digital printer or a digital copying machine which expresses gradation by modulating light intensity.
2. Description of the Prior Art
In a digital image forming apparatus such as a digital printer or a digital copying machine which forms an image by modulating a light intensity of a laser beam, a photoconductor is exposed in raster scan with a laser beam at an intensity modulated according to image data of a document read by an image reader. In the raster scan, the photoconductor is exposed during a time in correspondence to a length of one dot. Thus, each dot is formed in correspondence to an amplitude of image density data, and an electrostatic latent image is formed on the photoconductor.
The modulation of laser intensity can form an image with a high resolution, while it can form an image with smooth gradation characteristic. However, this method has a disadvantage in that pit noises along the subscan direction is liable to occur to deteriorate image quality. Further, gradation correction needed to correct the nonlinear gradation characteristic between image density on a document and output image density on a paper is affected by environment conditions and the like. Especially for a color image forming apparatus, image quality and gradation correction are important because a document including a color photograph is reproduced usually.
In order to solve the problems, U.S. patent application Ser. No. 07/971,055, now U.S. Pat. No. 5,420,614 discloses that an interval is provided between light emitting periods or the laser beam irradiates the photoconductor intermittently. By providing such intervals, the above-mentioned pitch noises do not become noticeable and this improves the smoothness of an image. Further, the nonlinearity of gradation characteristic which also affects image quality is also improved, and the image can be reproduced more stably. A duty ratio, defined as a ratio of a light-emitting period to a sum of the light-emitting period and a non-light-emitting period, can be changed. It is also disclosed that the above-mentioned interval is provided in the unit of a plurality of dots, say two dots.
Though the introduction of duty ratio improves the image quality and gradation characteristic as mentioned above, the image quality also depends on a kind of image such as a photograph image, and gradation characteristic is affected by environment such as humidity. Therefore, it is desirable that an image quality is improved more by taking them into account.